    Patent Document 1: JP-A H09-111000    Patent Document 2: JP-A 2004-225028    Patent Document 3: JP-A 2002-275272    Patent Document 4: JP-A 2007-147900    Patent Document 5: JP-A 2006-183018
In recent years, in the fields of paint, ink, adhesive, and the like, conversion in the composition is taking place from an organic solvent type to an aqueous type in view of such aspects as resource saving, environmental hygiene, pollution-free, and less danger. As the examples of vehicles used in an aqueous paint composition, there may be mentioned such resins as alkyd resin, acrylic resin, polyester resin, polyurethane resin, and epoxy resin. As the examples of transparent resins usable in optical applications such as an organic glass and a plastic lens, there may be mentioned such resins as methacrylic resin, polycarbonate resin, styrene resin, and epoxy resin. An aqueous emulsion formed by dispersing resin microparticles into water can be used in various uses, e.g., for resin particles in cosmetics and toners in electrophotograph, because of the spherical shape of the resin particle.
Among them, polyester resins represented by polyethylene terephthalate are excellent in such as mechanical, thermal, and electrical properties, and thus they are widely used in such articles as films, sheets, and bottles for various applications, whereby their demands are expanding. To produce such resin microparticles, as described in Patent Document 1, a method in which a solution containing dissolved resin in an organic solvent is preliminarily mixed with water and then the obtained mixture is emulsified with a mechanical energy in a batch-wise manner, is known. As described in Patent Document 2, a method for producing polyester resin microparticle aqueous dispersion, wherein a swelling material swollen in an organic solvent having a boiling temperature of below 100° C. with which the polyester resin can be swollen but is insoluble is mixed with an aqueous solvent containing a basic compound thereby neutralizing a part or all of the carboxyl group in the polyester resin by the basic compound while dispersing the swelling material into the aqueous solvent in the state of microparticles, and then the organic solvent is removed, is known. Further, as described in Patent Document 3 and Patent Document 4, a method to produce resin microparticles, wherein a molten resin is preliminarily mixed in an aqueous solvent and then a mechanical shear force is applied to the mixture thus obtained, is known.
However, with the methods as mentioned above, it has been difficult to obtain monodisperse and uniform particle diameter and particle size distribution, and in addition, raw materials usable for them have been limited. Furthermore, when an organic solvent is used, there is an absolute necessity of a step of solvent-removal after the step of emulsification/dispersion of the resin, which may be highly probable to result in complicated process and prolonged time for production. When resin microparticles are obtained by applying mechanical shear force to the molten resin, especially when the resin is molten and emulsified/dispersed in an aqueous solvent, treatment under normal pressure is difficult, whereby treatment under high temperature and high pressure becomes an absolute necessity. Naturally, this leads to a high danger, and in addition, specification of the equipment is for high temperature and high pressure, thereby tending to heavy equipment whose operation is difficult. Further, when the resin is molten under high temperature and high pressure, especially in the process in which the molten resin is preliminarily mixed in an aqueous solvent and then emulsified/dispersed, hydrolysis of the resin often occurs often because of long residence time in water at high temperature and application of mechanical shear force. All of these problems are caused by nonuniformity of the agitation in the reactor. That is, uniform distribution of concentration and temperature in the reactor is difficult to be established, and thus a complicated process, a prolonged residence time at high temperature and high pressure, excessive mechanical energy, a heavy equipment and hydrolysis accompanied by them, and so the like, would be resulted as mentioned above. Such problems may be solved by conducting the foregoing emulsification/dispersion in a microreaction field whereby temperature, concentration, and mixing state are strictly controlled, thereby enabling to accomplish uniformity of particle size and unification of reaction product effectively and efficiently.
Described in Patent Document 5, production methods of kinds of microparticles using various microreactors and micromixers are informed. There are many advantages in microreactors and systems thereof, but as the micro flow path diameter is decreased, a pressure loss is inversely proportional to the biquadrate of the flow path. That is, such high feeding pressure is necessary that a pump making possible to feed a fluid cannot be available. In the case of a reaction accompanied by separation, there is a problem that a microwave flow path is blocked by clogging of a flow path with a product or bubbles the reaction generates. Further, it is also a problem that since the reaction fundamentally depends on speed of molecular diffusion, a microscopic space is not effective or applicable to every reaction, and actual attempts of the reaction are required by trial and error, then good results are selected. Scaling up has been coped with a method of increasing the number of microreactors, that is a numbering-up system, but the number of microreactors which can be comprised is limited to several dozen, thus inherently aiming exclusively at products of high value. The increase in the number of devices leads to an increase in the absolute number of failure causes, and when the problem of clogging or the like actually occurs, it can be very difficult to detect a problem site such as a failure site.
In view of the above-mentioned, the present invention attempts to obtain resin microparticles in a thin film fluid formed between processing surfaces arranged opposite so as to be able to approach to and separate from each other, at least one of which rotates relative to the other, and has an object to provide a method for producing a resin microparticle aqueous dispersion, which can produce resin microparticles having monodispersity depending on its purpose owing to high uniformity of temperature in the thin film fluid and of the agitation in the reactor, and does not cause clogging with a product by self-dischargeability, and does not require a high pressure, and has a high productivity. An object further is to provide a method for producing a resin microparticle aqueous dispersion, which can produce uniform resin particles with a low energy and can remove a solvent more conveniently than by conventional methods. An object further is to provide a method for producing a resin microparticle aqueous dispersion, which enables to conduct the emulsification/dispersion treatment continuously in a short time, and because of its small holding quantity the equipment can be downsized thereby securing easy handling and high safety even when the molten resin is emulsified/dispersed at a high temperature. An object further is to provide a method for producing resin microparticle aqueous dispersion, which can produce an intended particle size distribution without the preliminary mixing, with a reduced risk of hydrolysis by a short treatment time, and with low energy.